1. Field of the Invention
The present invention relates to a pole-piece unit, a method for assembling the same, and a magnetic field generator. More specifically, the present invention relates to a pole-piece unit, a method for assembling the same, and a magnetic field generator used for an MRI apparatus and so on which requires an intense magnetic field greater than 0.3 T.
2. Description of the Related Art
In this kind of large magnetic field generator, e.g. a magnetic field generator for an MRI, in order to reduce magnetic flux leakage from pole pieces and to concentrate the magnetic flux effectively onto a space between the pole pieces, the applicant of the present invention proposed an arrangement, as disclosed in the Japanese Utility Model (of examined Application for opposition) No. 2-49683, in which a permanent magnet for repelling the magnetic flux leakage is provided on an outside surface of an annular projection.
When an intense magnetic field is required in this related art, a R—Fe—B magnet which has an intense magnetic power is used as the permanent magnet. In this case, the permanent magnet to be bonded comes under an intense repelling force when brought near the permanent magnet which is already bonded. Thus, it is difficult to bond the permanent magnet to the annular projection which is formed as a single piece, resulting in a poor production efficiency.
Further, according to the above related art, the permanent magnet for repelling the magnetic flux leakage is a magnet magnetized in a direction in parallel to a main surface of a permanent magnet used for generation of the magnetic field. If a uniform space is to be enlarged, the amount of magnet must be increased, causing a problem of increased cost.